The present invention relates to an optical star coupler for dividing optical beams transmitted through optical fibers between a plurality of optical fibers and a method of manufacturing the optical star coupler.
Accompanying the rapid progress of fiber optic transmission technology, the research and development of optical data links using optical fibers in data transmission between an electronic computer and another electronic computer or between an electronic computer and a terminal have been actively pursed. For constructing the optical data links, an optical star coupler which can mix the optical signals from a plurality of input optical fibers and divide them equally between a plurality of output optical fibers at low loss is an indispensable device.
As a typical optical star coupler, there so far has been a biconically tapered type as shown in FIG. 1. Such biconically tapered optical star coupler is shown, for instance, in (i) "Optical Communication Handbook" edited by Hisayoshi Yanai and issued by Asakura-Shoten on Sept. 1, 1982, pages 324 and 325, (ii) T. Ozeki et al. Electronics Letters, Vol. 12, No. 6 (1976), pages 151 and 152, and (iii) E. G. Rawson et al. Electronics Letters, Vol. 14, No. 9 (1978), pages 274 and 275, it is accordingly well known. This will be explained hereunder by using FIG. 1. This optical star coupler, in which multiple optical fibers 1 are bound in one portion and are provided with "twisting" and fused and stretched during heating to form a tapered region 5 in the central portion thereof, divides the optical signals from the input optical fiber (at the left of the tapered region 5) between a plurality of output optical fibers (at the right of the tapered region 5). The arrows in FIG. 1 indicate the direction in which the optical signals propagate. In this biconically tapered optical star coupler, it is reported that a favorable characteristic is obtained in the case that multi-mode optical fibers are used and the number of optical fibers is four or more. However, there are fewer reports for the case that single-mode optical fibers allowing long-distance and large-capacity transmission are used and that the numbers of the input and output ports are constructed in the ratio of 2:2, and it can not be said that the characteristics as shown in such reports are favorable. For the study by the present inventors, it turned out that it is particularly very difficult to realize an equally divided optical star coupler by using single-mode optical fibers.
That is, the present inventors repeatedly experimented with the construction as shown in FIG. 2 varying the twisting number, pulling length and fusing extent of the twisting, fusing and pulling portion 7 (corresponding to the tapered region 5 in FIG. 1). In FIG. 2, li designates input optical fiber and lo designates output optical fiber. But, an optical star coupler of low insertion loss and less power deviation cannot be realized. An attempt to obtain low insertion loss will produce extremely great power deviation of 10 dB or more, and an attempt to achieve less power deviation will make insertion loss to be 10 dB or more, thereby acting against one another. In addition, the outer diameter (clad diameter: 125 .mu.m) of each optical fiber corresponding to the twisting, fusing and pulling portion 7 was made 20 .mu.m by etching, and the cores in the respective optical fibers (core diameter: about 10 .mu.m, necessary to be made as small as this to realize single-mode optical fiber) were made close to each other to try to achieve tight coupling. In this case, power deviation could easily be reduced for the optical fiber diameter that is on the order of 20 .mu.m, but, conversely, it proved that the twisting, fusing and pulling portion 7 broke or bent in the center thereof in the twisting, fusing and pulling process because of the small optical fiber diameter and this lowered the reliability to a great extent. Eventually, in consideration of this point, the outer diameter of an optical fiber which can be handled by this method was on the order of 60 .mu.m at the minimum. However, for the optical fiber diameter on the order of 60 .mu.m, sufficient coupling could not be achieved because of too wide a space between the cores, and power deviation for single-mode optical fiber was 10 dB or more for 6 dB or less insertion loss.
From the reason described above, it turned out that, in the biconically tapered optical star coupler, application of a single-mode optical fiber allowing long-distance and large-capacity transmission is difficult if a twisting, fusing and pulling portion is merely formed and that application to multi-mode optical fiber having the ratio of 2:2 of the mumbers of input and output ports is also difficult.
Further, in the manufacturing aspect, as described above, it is necessary to pull the outer diameter of the tapered region 5 as thin as a dozen .mu.m to a hundred and several tens of .mu.m in the forming of a single-mode optical fiber, and, at this time, the pulled portion can be damaged or it can break when it is handled. In addition, an oxyhydrogen burner is usually used as the heating source which is utilized for pulling, there is a problem that the fused portion can be bent or deformed by the wind pressure of the oxyhydrogen burner, or it can be excessively fused so that a outer diameter cannot be controlled to the desired value, and the yield rate of manufacturing will accordingly be very bad. Particularly, when the input and output port numbers are small, it is necessary to make the outer diameter of the above region some dozen .mu.m to several tens of .mu.m (this value varies according to the clad thickness), so the above problem is more serious.
Moreover, OH ions or transition metal ions mix into the twisting, fusing and pulling portio from the flame of the oxyhydrogen burner and the air in the atmosphere, causing optical absorption loss.
As a prior art optical power divider (optical star coupler) of 2 to 2 type using single-mode optical fiber, some examples are shown in FIGS. 3a, 3b, 4a and 4b. In FIGS. 3a and 3b, optical fibers 1a and 1b are respectively embedded in blocks 31a and 31b, and adhesively fixed, and planes 32a and 32b are put together after they are ground. FIG. 3b shows the enlarged A--A' section in FIG. 3a. By doing so, the space between the respective cores 2a and 2b of the optical fibers 1a and 1b is made very close thereby to enhance the coupling efficiency. However, grinding thickness controllability is poor and a long manufacturing time is required, thus, the manufacturing cost becomes extremely high. That Numerals 3a and 3b in FIG. 3b designate the clads of the optical fibers 1a and lb, respectively.
In FIGS. 4a and 4b, optical fibers 33a and 33b having cores with concentricity error are used to make the space s (several .mu.m) between the respective cores 2.sub.33a and 2.sub.33b closer. FIG. 4b shows the enlarged section A--A' of FIG. 4a. In this construction, however, it is difficult to make the optical fibers 33a and 33b having cores with concentricity error, and the manufacturing cost will rise. In addition, when the core space is made minimum to perform the fusing, it cannot be determined whether it is made minimum, so the manufacturing yield rate becomes a problem.
As already described above, an oxyhydrogen burner or an electric furnace is usually used as the heating source of optical fibers. By the method using an oxyhydrogen burner, the temperature can be increased or decreased within a short time, but the twisting, fusing and pulling portion is deformed easily by the wind pressure of the burner,flame temperature and disturbance. Namely, the form control of the above twisting, fusing and pulling portion is almost impossible, thus, the yield rate is very bad and there is a problem in reproducibility and mass-productivity. The method using an electric furnace can control the above disturbance, but the temperature increases or decreases very slowly so that there is a problem that optical fibers cause undesirable deformation during the increase or decrease of the temperature. As described above, it is difficult to make an optical star coupler having favorable optical characteristics at a good yield rate by using the prior art. In addition, the yield rate becomes very bad, so it is also difficult to lower the cost
As described above, it was difficult to realize an optical star coupler for single-mode optical fiber permitting long-distance and large-capacity transmission, an equally divided single-mode optical star coupler having the ratio of n:n input and output port numbers, and further multi-mode optical star coupler having the ratio of n:n input and output port numbers with low insertion loss, low power deviation and low cost. The above described "n" means 2 or more.
Recently, as shown in FIG. 5, such construction (hereinafter referred to as optical star coupler connected without any splicing) as connecting the output port fiber 130-3 and the input port fiber 131-1 of two optical power dividers 133 and 134 in tandem connection (connection part 132) is beginning to be utilized in various systems. In this case, an attempt to make the construction by using two such optical star couplers as shown in FIG. 1 or FIG. 2 will produce connection loss at the connection part 132, increasing the overall loss. In addition, if the connection is performed by the thermal splicing method or the like, it is necessary to make the lengths of the fibers 130-3 and 131-1 sufficiently long, and the loss will increase to that extent. The size of the optical power divider also increases. Further, an additional process such as thermal splicing work increases the cost. As described above, it proved that the construction provided by combination of separately made optical power dividers includes various problems.
In FIG. 5, numerals 130-1, 130-2, 130-4, 131-2, 131-3 and 131-4 designate input and output port fibers.